Phosphorus has been identified as main responsible for eutrophication in lake or river waters. Therefore, phosphorus removal is a special concern in the treatment process lines of the waste water treatment plants to minimize its release into the environment.
To this effect, urban and industrial waste water treatment plants uses generally chemical precipitation steps, for instance flocculation-coagulation steps by iron or aluminum salts, or by a more complex process called Enhanced Biological Phosphorus Removal (EBPR), often complemented by chemical precipitation to reach the legal limits.
However, chemical precipitation, yet the most used technique, increases the sludge production, its toxicity and its disposal costs. EBPR also produces large volumes of sludge containing phosphorus. To reduce the sludge volume, and/or produce energy, said sludge may be digested under anaerobic conditions at a pH above 6.5 to produce methane.
Iron salts, such as ferric chloride, are mostly used for these precipitation steps, but are easily combined with phosphorus to generate a precipitate of iron phosphate FePO4. Sludge issued from these waste water plants are thus charged both in iron and phosphorus. Iron present in said sludge combines with phosphorus which is therefore less available as agricultural fertilizer after sludge landfill disposal.
Recently it has been tried to extract phosphorus from said sludge to increase its value, particularly in the form of a magnesium ammonium phosphate hexahydrate, MgNH4PO4.6H2O, called struvite, that can be used as fertilizer. However, the presence of iron inhibits the crystallization of struvite and thus impairs the recovery of phosphorus.
In addition, it also appears that struvite could be spontaneously formed inside pipes and other equipments. Such crystallization should be controlled and avoided.
A first aim of the present invention is to remove iron from sludge of waste water treatment plants, said sludge being obtained after precipitation by iron salts, combined or not with EBPR (and having or not undergone an anaerobic digestion step), in order to allow phosphorus recycling.
Another aim of the invention is to propose a method of recovering both iron and phosphorus from said sludge of waste water treatment plants after precipitation of sludge by iron salts.